This invention relates in general to the control of steam quality from a once-through forced circulation boiler supplying steam and water mixtures to an oil field for enhanced oil recovery.
Steam injection is used in the oil industry to promote the flow of viscous, heavy oils or liquid hydrocarbons from tar sands to producing wells by heating the deposits and reducing their viscosity. Currently, practically all steam generators for this service are small, portable, once-through type units fired with oil or gas. They generally utilize available, untreated water at the site which contains high concentrations of dissolved solids. Small size units consist of a single tube formed into a helix or having a serpentine flow path. Steam quality leaving these monotube boilers is controlled to about 90% steam by weight. Larger units consist of three or more tubes in parallel. Flow to each circuit is metered and controlled for an average outlet quality of about 80% steam by weight. The lower quality steam and consequent higher water flow in the output of the multi-tube unit compared to a monotube design is to provide an added factor of safety for flow unbalances and upset heat absorption rates. It is vital to successful operation of these once-through units utilizing untreated feedwater to prevent evaporation to dryness in any tube which could result in the deposition of solids in that tube, leading to tube failure and a forced outage.